The 6.5L GM Turbo Diesel engine, a robust and reliable workhorse, was a popular choice in Chevrolet and GMC trucks and SUVs throughout the mid-1990s. For models produced between 1993 and 1995, these engines utilized the first generation of on-board diagnostics, known as OBD I. While less sophisticated than the OBD2 systems that became standard in later vehicles like some 1998 models, understanding the OBD I system is crucial for diagnosing and maintaining these classic diesel engines.
This guide provides a comprehensive overview of the OBD I diagnostic system specific to the 1993-1995 6.5L GM Turbo Diesel engine. We will cover everything from locating the diagnostic connector to reading and interpreting the Diagnostic Trouble Codes (DTCs) that can help you pinpoint potential issues. While later 1998 models and beyond transitioned to OBD2, mastering OBD1 is essential for owners and enthusiasts of these earlier, dependable diesel trucks.
OBD I Diagnostic System: An Overview for 6.5L Diesel Engines
General Motors adopted the OBD I system for the 6.5L diesel starting in 1993 and continued through the 1995 model year. It’s important to note that OBD I systems are not standardized across different vehicle manufacturers like the later OBD II. OBD II, with its enhanced monitoring capabilities and standardized codes, became the industry benchmark from 1996 onwards. However, for owners of these earlier 6.5L turbo diesel vehicles, OBD I is the diagnostic system you need to understand.
The primary function of the OBD I system is to alert drivers to potential malfunctions, primarily within the engine and transmission systems. When a fault is detected, the system stores a DTC, which is a numerical code corresponding to a specific problem. These DTCs are valuable clues for technicians and DIY mechanics, helping to narrow down potential issues and guide troubleshooting efforts. Keep in mind that a single engine problem might trigger multiple DTCs, each representing a piece of the diagnostic puzzle.
When the “Service Engine Soon” light illuminates on your dashboard – typically located to the left of the tachometer – it signifies that at least one DTC is stored within the OBD I system. The beauty of the OBD I system in these 6.5L diesels is that you can retrieve these DTCs without needing a specialized scan tool. By initiating a simple on-board diagnostic sequence, you can access the stored codes and begin your diagnostic journey.
Finding the OBD-I Connector on Your 6.5 GM Diesel
To access the OBD I system, you’ll need to locate the Assembly Line Diagnostic Link (ALDL) connector. For 6.5 GM diesel vehicles of this era, the 12-pin ALDL connector is typically positioned beneath the dashboard, to the left of the steering column.
Figure 1 – Location of the OBD I diagnostic connector in a 6.5 diesel GM vehicle.
If you don’t immediately see the connector hanging below the dash, it’s worth checking behind the dashboard. Over time, the mounting tabs can break, and the connector might be tucked away with the wiring harness behind the lower dash panel.
OBD-I Connector Pin Configuration
Initiating the diagnostic sequence on the OBD I system involves connecting terminals “A” and “B” on the ALDL connector. This process, often called “jumping” the terminals, can be accomplished with any conductive material. A small gauge copper wire, with its ease of handling, is a practical choice for this task.
Figure 2 – Pinout diagram illustrating the terminal layout for the 12-pin GM OBD I connector used in 6.5 diesel engines.
Decoding DTCs on a 6.5 Diesel Engine
Once you’ve initiated the diagnostic sequence by jumping terminals A and B, the “Service Engine Soon” (SES) light will become your communicator. DTCs are conveyed through a flashing pattern of the SES light, similar to Morse code. You’ll need to carefully observe and count the flashes to decipher the codes.
Short pauses between flashes indicate that the system is still within the same digit of a code. A longer pause signifies the completion of a digit and the start of the next. To illustrate, let’s use a dash (“-“) for a short pause, three dashes (“—“) for a long pause, and “SES” to represent a brief illumination of the Service Engine Soon light:
- SES—SES-SES: This sequence represents the code “12”. The long pause separates the digits, and the flashes within each group indicate the numerical value.
- SES-SES—SES-SES-SES: This translates to the code “23”.
- SES-SES-SES—SES-SES-SES-SES-SES: This sequence represents the code “35”.
Figure 3 – The “Service Engine Soon” light on a 6.5 diesel instrument cluster, your key to reading OBD-I diagnostic trouble codes.
All stored DTCs will be displayed numerically in ascending order and each code will repeat three times before the system moves on to the next stored code. The sequence always begins with DTC 12, which is a system check code confirming that the diagnostic system is functioning correctly. DTC 12 will flash three times at the beginning of each diagnostic cycle. If no other DTCs are stored, the system will continuously repeat the DTC 12 sequence.
Here’s a step-by-step guide to entering diagnostic mode and reading DTCs:
- Ensure the ignition switch is in the “OFF” position. Then, use a short length of wire (16 AWG solid core wire stripped at both ends works well) to connect terminals “A” and “B” on the OBD I connector, as shown in Figure 2.
- Turn the ignition switch to the “RUN” position. This will initiate the on-board diagnostic code retrieval sequence. Do not start the engine.
- Carefully observe the “Service Engine Soon” light. It will first display DTC 12 three times, followed by any other stored DTCs, also repeated three times each.
- Once you have recorded all the codes, turn the ignition switch back to the “OFF” position and disconnect the wire jumper from terminals “A” and “B”.
The diagnostic sequence will continue to loop as long as the key is in the “RUN” position (engine off) and terminals A and B remain connected. If you miss a code or need to re-verify a DTC, simply allow the sequence to cycle again – it will always restart with three flashes of DTC 12. If the SES light fails to illuminate at all, double-check that terminals A and B are securely connected. Other potential causes could be a burned-out SES light bulb or wiring issues within the OBD I connector circuit.
Key points to remember when reading OBD-I codes:
- Count the SES light flashes. Short pauses separate digits within a code; a long pause indicates the end of a digit and the start of the next.
- Each DTC is displayed three times in a row before proceeding to the next code.
- The diagnostic sequence always begins with DTC 12, repeated three times. This is a system check code.
- If only DTC 12 repeats continuously, it indicates that no other trouble codes are currently stored.
- DTCs range from 12 to 99. There are no single-digit codes, and some numbers within this range are not used.
- Codes are always displayed in numerical order, from lowest to highest.
6.5 Diesel OBD-I Diagnostic Trouble Code (DTC) List
Below is a comprehensive list of OBD-I DTCs for the 1993-1995 6.5L Turbo Diesel engine. This list will help you understand the meaning of the codes you retrieve from your vehicle’s diagnostic system.
- 12 – Diagnostic system functioning properly (check code)
- 13 – Engine shutoff solenoid circuit fault
- 14 – Engine coolant temp sensor circuit low (high temp)
- 15 – Engine coolant temp sensor circuit high (low temp)
- 16 – Vehicle speed sensor buffer fault
- 17 – High resolution circuit fault
- 18 – High resolution circuit fault
- 19 – Crankshaft position reference fault
- 21 – Accelerator pedal position 1 circuit high
- 22 – Accelerator pedal position 1 circuit low
- 23 – Accelerator pedal position 1 circuit out-of-range
- 24 – Vehicle speed sensor circuit low
- 25 – Accelerator pedal position 2 circuit high
- 26 – Accelerator pedal position 2 circuit low
- 27 – Accelerator pedal position circuit out-of-range
- 28 – Transmission range pressure switch circuit fault
- 29 – Glow plug relay fault
- 31 – EGR control pressure/barometric sensor circuit low
- 32 – EGR circuit fault
- 33 – EGR control pressure/barometric sensor circuit high
- 34 – Injection timing stepper motor fault
- 35 – Injection pulse width error, short duration (pulse width shorter than desired)
- 36 – Injection pulse width error, long duration (pulse width longer than desired)
- 37 – Torque converter clutch (TCC) brake switch fault, stuck “ON”
- 38 – Torque converter clutch (TCC) brake switch fault, stuck “OFF”
- 39 – Torque converter clutch (TCC) stuck “OFF”
- 41 – Brake switch circuit fault
- 42 – Fuel temperature circuit low (temp high)
- 43 – Fuel temperature circuit high (temp low)
- 44 – EGR pulse width error
- 45 – EGR vent error
- 46 – Malfunction indicator lamp (MIL) circuit fault
- 47 – Intake air temperature sensor circuit low (temp high)
- 48 – Intake air temperature sensor circuit high (temp low)
- 49 – Service throttle soon lamp circuit fault
- 51 – P.R.O.M. error
- 52 – System voltage high, long
- 53 – System voltage high
- 54 – PCM fuel circuit error
- 56 – Injection pump calibration resistor error
- 57 – PCM 5 volt shorted
- 58 – Transmission fluid temperature circuit low (high temp)
- 59 – Transmission fluid temperature circuit high (low temp)
- 61 – Turbocharger boost sensor circuit high
- 62 – Turbocharger boost sensor circuit low
- 63 – Accelerator pedal position 3 circuit high
- 64 – Accelerator pedal position 3 circuit low
- 65 – Accelerator pedal position 3 circuit out-of-range
- 68 – Transmission slip detected
- 69 – Torque converter clutch (TCC) stuck “ON”
- 71 – Cruise control set/coast switch fault
- 72 – Vehicle speed sensor circuit loss
- 73 – Pressure control solenoid circuit error
- 74 – Transmission input speed sensor circuit fault
- 75 – System voltage low
- 76 – Cruise control resume/accelerate switch fault
- 78 – Turbocharger wastegate solenoid fault
- 79 – Transmission fluid over temperature
- 81 – Transmission 2-3 shift solenoid circuit fault
- 82 – Transmission 1-2 shift solenoid circuit fault
- 83 – Torque converter clutch fault
- 84 – Accelerator pedal position circuit fault
- 85 – Transmission gear error
- 86 – Transmission gear ratio error
- 87 – Transmission gear ratio error
- 88 – Top dead center offset error
- 91 – Cylinder balance fault, cylinder 8
- 92 – Cylinder balance fault, cylinder 7
- 93 – Cylinder balance fault, cylinder 6
- 94 – Cylinder balance fault, cylinder 5
- 95 – Cylinder balance fault, cylinder 4
- 96 – Cylinder balance fault, cylinder 3
- 97 – Cylinder balance fault, cylinder 2
- 98 – Cylinder balance fault, cylinder 1
- 99 – Accelerator pedal position sensor reference voltage error (5 volt ref)
Conclusion: OBD-I as a Diagnostic Starting Point
While OBD-I is less advanced than today’s OBD-II systems, it remains a valuable tool for diagnosing potential powertrain issues in 1993-1995 6.5L turbo diesel GM vehicles. Understanding how to access and interpret OBD-I DTCs is a crucial first step in troubleshooting engine and transmission problems. Remember that these codes are intended to guide your diagnostic process, offering a starting point for further investigation and testing to accurately pinpoint the root cause of any issue. By using this guide and the provided code list, you can confidently begin diagnosing your classic 6.5 diesel and keep it running smoothly for years to come.
